In a wheel support bearing device of a kind to which the present invention pertains, it is well known that the wheel hub has a wheel mounting flange formed integrally therewith so as to extend radially outwardly therefrom so that a vehicle wheel can be removably secured thereto by means of a plurality of bolts. This wheel mounting flange has a root portion integral with the wheel hub, which generally tends to be subjected to considerable stresses particularly when an automotive vehicle makes an abrupt turn. Accordingly, in order to increase the fatigue strength for the purpose of avoiding fracture of the root portion of the wheel mounting flange relative to the wheel hub, such root portion is generally treated with an induction heat treatment or an induction hardening such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2004-182127 or a shot peening such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2005-145313. Also, in order to increase the fatigue strength, a method has been suggested, in which a component part is in its entirety thermally refined such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2005-003061.
FIG. 14 illustrates a generally employed method of making a wheel hub used in a conventional wheel support bearing assembly of a third generation type. This generally employed method includes cutting a bar WO to a predetermined size as shown by (A) in FIG. 14 to thereby provide a billet W1, as shown by (B) in FIG. 14, which eventually serves a raw material for one wheel hub. The billet W1 is then passed through a plurality of passes forming respective steps of a hot forging process, which passes include, for example, a first forging pass, a second forging pass and a third forging pass, to allow it to eventually assume a shape similar to the shape of the wheel hub, followed by a final forging process by which a finally forged product W4 of a shape approximately similar to the shape of the wheel hub can be obtained. See (C) to (E) in FIG. 14.
The finally forged product W4 is treated with a shot blasting for the purpose of removal of surface scales and is subsequently normalized or thermally refined if so required, as shown by (F) in FIG. 14. Then, the finally forged product W4 is subjected to turning as shown by (G) in FIG. 14, followed by induction heat treatment, as shown by (H) in FIG. 14, applied to raceway surfaces or the like. Also, if required, a secondary turning is carried out subject to flange surfaces or the like as shown by (I) in FIG. 14. Thereafter, grinding is effected to finish the wheel hub 14, which is subsequently assembled to complete a wheel support bearing device.
It may often be experienced that the induction heat treatment hitherto employed to increase the fatigue strength cannot be employed satisfactorily depending on the shape of a component part where the induction hardening is applied. For example, such cases are observed that in view of the fact that the root portion of the wheel mounting flange referred to above has its side surface, from which a pilot portion for guiding a brake and/or a vehicle wheel protrudes, the radius of curvature of a corner between the flange and the pilot portion tends to be small and/or the pilot portion represents a plurality of prongs that are dispersed in a direction circumferentially thereof through corresponding cutouts. In the case of such shape, a problem may occur that a portion of a component part, when locally heated to an elevated temperature as a result of the induction hardening, may melt down and, accordingly, no induction hardening cannot be employed.
Also, in the practice of the induction hardening and the shot peening, it may occur that the number of process steps tends to increase and/or the run-out precision of the flange may be lowered.
In the case of a method, in which the component part in its entirety is thermally refined to increase the hardness, the number of process steps increases. Also, an increase of the hardness results in reduction of the processability (for example, the capability of being machined, and the cold workability such as, for example, a crimping process) of the entirety, and, accordingly, reduction of a slip torque that may be brought about as a result of insufficient press-fitting of hub bolts to respective bolt holes of the flange, for example, may occur.
With the conventional method shown and described with reference to FIG. 14, while the normalizing or thermal refining is carried out in order to increase the fatigue strength of the wheel hub 14 in its entirety, the production is complicated and time-consuming because of the increase in number of process steps including normalization or thermal refinement, and, further, the amount of energies consumed increases because of the necessity that the wheel hub 14 after forged and then cooled must be heated again. Although there is a case, in which the normalization or the thermal refinement referred to above may be dispensed with, the wheel hub may have a structure containing relatively large crystalline particles, have a reduced strength and a reduced toughness and have a low fatigue strength if the normalization or thermal refinement is dispensed with.
Also, in recent years, in order to increase the mileage and reduce the environmental loading, compactization and reduction in weight are strongly desired for even in the wheel support bearing device and, accordingly, it is necessary to achieve such compactization and reduction in weight while securing a high fatigue strength and a life time.